This disclosure pertains to a Schmidt telescope and in particular to a flat field Schmidt telescope having an extended field of view.
A classical Schmidt telescope, also referred to as Schmidt camera, is a catadioptric astronomical telescope or camera designed to provide relatively wide field of view while reducing aberrations in comparison with other telescopes. It is named for its inventor Bernhard Schmidt, who wrote of its properties in 1931 in “Ein lichtstarkes komafreies Spiegelsystem,” Central-Zeitung für Optik and Mechanik 52.2 (1931), pp. 25-26. A detailed description of a flat field Schmidt telescope can be found in “Recent Advances in Optics,” by E. H. Linfoot, Oxford University Press, 1955. FIG. 1 shows a flat field Schmidt telescope 10. Flat field Schmidt telescope 10 includes spherical primary mirror 12 and aspherical correcting plate (known as a corrector plate) 14 spaced apart from primary mirror 12. Aspherical correcting plate 14 is generally not located at a center of curvature of primary mirror 12 for flat field Schmidt telescopes, unlike classical Schmidt telescopes, but in both cases, the corrector plate generally defines the entrance pupil 15 of telescope 10. A film or other detector 16 is placed inside telescope 10 between primary mirror 12 and corrector plate 14, at a focal point of primary mirror 12. Flat field Schmidt telescope design 10 is noted as having fast focal ratios (i.e., small focal ratios or small F-numbers) while limiting coma and astigmatism.
A classical Schmidt telescope has a curved focal plane or curved field plane. As a result, a classical Schmidt telescope generally requires a curved detector to match the curvature of the focal plane. In order to be able to use a flat detector instead of a curved detector, field flattener or field flattening optics 18 can be placed in front of detector 16 between primary mirror 12 and detector 16 so as to obtain a flat field plane on detector 16. Flat field Schmidt telescopes such as Schmidt telescope 10 are typically used as a survey instrument in which a large area of the sky is covered.
A classical Schmidt telescope configuration can be modified by combining with a Cassegrain telescope configuration to produce the Schmidt-Cassegrain telescope. In the Schmidt-Cassegrain telescope, a Cassegrain reflector's optical path is combined with a Schmidt corrector plate (i.e., aspherical correcting plate) to make a compact telescope. The Schmidt-Cassegrain telescope uses a spherical primary mirror, a Schmidt corrector plate and a Cassegrain secondary mirror which can act as field flattener and directs the image through an opening in the primary mirror to a final focal plane located behind the primary mirror. In addition to the Schmidt-Cassegrain telescope, other combinations or modifications of the Schmidt telescope are also possible such as the Baker-Schmidt telescope, the Mersenne-Schmidt telescope, the Schmidt-Newtonian telescope, and the Schmidt-Väisälä telescope, etc.
Although the flat field Schmidt telescope 10 provides relatively wide field of view (FOV) compared to other types of telescopes, the field of view (FOV) of conventional flat field Schmidt telescope 10 is generally limited to approximately 8 to 9 deg. (e.g., about 8.6 deg.). In addition, the flat field Schmidt telescope 10 is mostly used in the visible range of wavelengths. The corrector plate or correcting lens 14 and the field flattening optics 18 are made from optical glass or fused silica which have a refractive index between about 1.5 and about 1.6 and/or Calcium Fluoride (CaF2) which has a refractive index of about 1.4. These materials transmit light in the visible range of wavelengths.
Therefore, there is a need in the art for a flat field Schmidt telescope that is configured to be used in the infrared range of wavelengths including the short wavelength infrared (SWIR) range (between about 1.4 μm and about 3 μm) and the mid wavelength infrared (MWIR) range (between about 3 μm and about 8 μm) while providing an extended field of view (FOV).